1. Field of the Invention
The invention relates to improvements in methods and apparatuses for reducing vibration-induced noise in machinery, and in particular to a method and apparatus for actively cancelling vibrations in a structure that supports rotating machinery.
2. Prior Art
It is desirable to reduce or eliminate vibrations induced in rotating machinery. Various active and passive methods have been employed to suppress vibrations. Examples of passive methods include cushion supports and mechanical damping means which in essence absorb and dissipate the vibrational energy produced by the disturbance. Passive methods are generally unsatisfactory because the vibrational energy is ultimately transferred to the environment. This occurs because the vibrational energy contains complex wave forms which manifest themselves at various harmonics of the fundamental vibration frequency. A simple mechanical absorber or damper may thus be essentially transparent to the various components of the vibrational energy. Accordingly, such efforts to suppress, cancel or eliminate the vibrations may not be effective.
Active methods are more successful at eliminating or cancelling vibrations. However, these too have deficiencies. Like passive methods, active methods may only be operable within a narrow frequency range of the fundamental distrubance. In active systems a counteracting force is produced which opposes the force produced by the disturbance. The opposing force is not easily generated with accuracy because the nature of the disturbance is rarely completely known. The problem is further aggravated by the fact that the structure may be complex and is not amenable to a simplified rigid body analysis. Furthermore, most active systems must be custom designed for a specific structural system and/or type and postion of forcers in that the specific characteristics of the physical system as well as those of the forcers must be known. For example, U.S. Pat. No. 4,626,754 to Habermann et al. discloses a method and device for reducing the vibrations of rotating machines with an active suspension which is limited to machines which utilize electromagnetic bearings that have electromagnets arranged in pairs along fixed orthogonal diametral axes.